As shown in FIG. 1, an axial flow gas turbine turbofan engine 10 comprises an air intake 11, a low pressure compressor (or fan) 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18, and an exhaust nozzle 19. A nacelle assembly 20 encloses the fan 12.
In operation, air is drawn into the engine 10 through the intake 11 and accelerated by the fan 12, to produce two air flows: a first air flow which enters the intermediate pressure compressor 13 and a second air flow which bypasses the core of the engine to provide direct propulsive thrust.
The ratio between the mass flow rates of these first and second air flows is termed the bypass ratio.
The first air flow entering the intermediate pressure compressor 13 is compressed before entering the high pressure compressor 14 where further compression takes place.
The compressed air leaving the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the resulting mixture is combusted. The high pressure combustion products then rapidly expand as they pass through and drive the high, intermediate and low pressure turbines 16, 17 and 18. The gas leaving the low pressure turbine 18 is then exhausted through the exhaust nozzle 19 and provides additional propulsive thrust.
The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by means of separate interconnecting shafts.
It is known that increasing the bypass ratio of a turbofan engine can reduce its fuel consumption and consequent level of CO2 emissions. This characteristic has been exploited by engine manufacturers by progressively increasing the bypass ratios of modern turbofan engines.
However, there is a limit to how much the bypass ratio can be increased as eventually the weight and drag penalties associated with the size of the required engine nacelle outweigh the reduction in fuel consumption.
Most turbofan engines are designed to be capable of being mounted in an under-wing configuration. Such a configuration provides an upper limit to the nacelle diameter so as to maintain safe working ground clearance beneath the engine when it is installed on the aircraft.
Furthermore, turbofan engines typically have a relatively deep nacelle which encloses the fan and the core engine. This nacelle depth, in combination with the need to maintain a minimum safe ground clearance, limits the diameter of fan which can be employed on the engine.